What is Vapor Pressure and How does it Impact Pump Selection

May 14, 2014

Last week, we had a post that focused on two important fluid properties- density and specific gravity- and took a look at how those factors can affect pump selection. Today, we’ll look at another commonly overlooked fluid property, vapor pressure, and how it affects pump sizing.

Vapor pressure

Vapor Pressure of Water

To begin, there are three classic phases of matter- gas, liquids, and solids. The state of matter that we are generally pumping are fluids. Fluids will evaporate unless prevented from doing to by some external pressure. For most the fluids we see every day, like water, atmospheric pressure is generally enough to keep water from becoming vapor. The vapor pressure of a fluid is the pressure, at a given temperature, at which a fluid will change to a vapor. Each fluid has its own vapor pressure/temperature relationship.

So, you may ask, is the vapor pressure also the fluids boiling point? In short, yes, but the answer is not that simple. As a general trend, vapor pressures at ambient temperature increase with decreasing boiling points. To illustrate this point, let’s look at water. At 32 F, the vapor pressure of water is 0.087 psia. This means that for water at 32 F, if the external pressure drops below 0.087 psia (pretty serious vacuum), the water will flash off and boil. At 68 F (more or less ambient water temperature), the vapor pressure of water is 0.334 psia (still a pretty strong vacuum).

At 212 F, the vapor pressure of water is 14.7 psia, or atmospheric pressure.
Finally, before we get into how this affects pump sizing and selection, you may wonder if there is a similar point where a liquid will become a solid. There is- and this temperature is known as the fluids heat of fusion, enthalpy of fusion, or more generally as the melting point. In fact, there exists a point, where at a given temperature and pressure a substance can be solid, liquid, and gas. This point is known at the triple point of a substance.

Back to the point- how does vapor pressure affect pump selection? In pump sizing, vapor pressure is a key check in evaluating suction conditions and the net positive suction head available to the pump. In order to pump a fluid, we need to deliver fluid to the inlet of the pump. This can become a real issue with either self-priming pumps or when pumping hot fluids. Pumps work by creating an area of very low pressure near the inlet of the pump. Some pumps, like the Quattroflow quarternary diaphragm pump, will even suck and pump air. If the pump pressure at the inlet of the pump drops below the fluids vapor pressure, the fluid will flash off. This will wreak havoc on your system, causing cavitation, pump life deterioration, and other inefficiencies.

Another condition we have issues with this is with high temperature fluids. In liquid sugar or hot fluid applications, where the temperatures are sufficiently high to cause vapor pressure decrease, we can theoretically satisfy the NPSHR requirement of the pump, but not have enough pressure to keep the product liquid, resulting in cavitation.

To conclude, each of application should be evaluated independently and take into consideration not just fluid rheologic properties, but physical properties as well. Common things we can do to increase suction pressure and avoid cavitation include running pumps slower, increasing inlet line size, and using a booster pump to supply additional suction pressure. If you have a tricky sanitary pump application, don’t just call anyone, contact a Holland Sales Engineer today and we’ll get you sorted out.